This invention relates to a focussing control method for use in a laser optical disk, and more particularly, to a focussing control method which uses the astigmatic light flux of a diffraction grating, or a hologram in the digital data retrieval system of a laser optical disk.
Recently various kinds of apparatus using lasers have been developed, and applications of this apparatus are expanding in various fields, for example, in the optical video disk field.
In an optical video disk, the light beam recording layer of the disk recording medium is illuminated with a converged laser beam to record audio information, image information, or digital information on the recording layer whose material comprises alluminum, tellurium, or metallic thin film. Since the beam spot with which the laser beam illuminates the recording medium to record various information thereon is as small as about 1 .mu.m, a very high density recording can be achieved.
Conversely, in converging the laser beam and illuminating the recorded layer for the retrieval of information, the narrowest part of the converged laser beam, i.e. the laser beam waist, has to be coincident with the light beam recorded layer. This means that the converged laser beam must be controlled so as to be applied to the recorded layer with a correct focal point.
In the conventional method, the focussing control of the laser beam is effected as follows:
The laser beam converged with a converging lens illuminates the recorded medium, and is reflected from the recorded medium with a beam splitter. The reflected laser beam strikes a cylindrical lens, which produces an astigmatic light flux. The astigmatic light flux is received on a light sensitive device.
The light sensitive device is divided into four equal areas with the optical axis at the center, forming two pairs of oppositely facing areas. At the position where the light sensitive device is provided, the reflected laser beam forms a round, vertically elliptical, or horizontally elliptical pattern. Thus, the size of the output signals from each pair of oppositely facing areas in the light sensitive device is compared to determine whether the laser beam converged on the recorded medium surface is in focus. When the reflected laser beam forms a round pattern, the size of the output signals from one pair of oppositely facing areas equals that of the output signals from another pair, indicating that the laser beam is converged in focus on the recorded medium. In case of a vertically elliptical or horizontally elliptical pattern being formed, the size of the output signals from each pair of oppositely facing areas differs from each other, indicating tht the laser beam converged on the recorded medium is out of focus. In such a case, the distance between the converging lens, and the recorded medium surface needs to be adjusted, by shifting the converging lens.
The output signals from the light sensitive device are compared, and the optical distance between the converging lens and the reflection surface is varied whereby the focus of the apparatus is controlled.
As described above, the conventional method uses a cylindrical lens as a means to produce an astigmatic light reflux. However, a cylindrical lens requires high manufacturing accuracy, and thus is expensive. In addition, a device using a cylindrical lens, due to its considerable thickness, is necessarily heavy.
Those problems can be solved, according to this invention, by using a diffraction grating or a hologram as a means of producing the astigmatic light flux.